The use of so-called safety glazing or penetration resistant glazing for windows, windshields and the like utilizing polycarbonate resin layers as a structural component is well known. For example, glass polycarbonate resin laminates are described in U.S. Pat. No. 3,666,614, the glass and polycarbonate being cohered together using an ethylene-vinyl acetate copolymer. In U.S. Pat. No. 3,520,768, there are described laminates of relatively thick glass having a comparatively thin polycarbonate foil on the cohering material. It is also known, as described in U.S. Pat. No. 4,027,072, to utilize certain polysiloxane-polycarbonate block copolymers as an adhesive in preparing polycarbonate containing laminates. It is normal practice in constructing certain such laminates to utilize glass or relatively hard solid resinous materials as the impact or shock receiving layers while utilizing polycarbonate as the back or downstream layer. In those cases where polycarbonate is used as a layer of a laminate, it is often, because of the relative softness of the polycarbonate, protected, especially on its exposed surface, with a mar- or scratch-resistant and transparency-preserving coating. Thus, U.S. Pat. No. 4,123,588 describes a laminate containing a back polycarbonate lamina coated with a scratch resistant-layer of microglass. In general, such mar-resistant coatings which are well known can be metal oxides; modified melamines; ultraviolet light hardenable organics such as acrylated monomers or mixtures of these monomers with acrylate-modified polymeric resins; inorganic glasses such as silica or alumina; polyurethanes; silicones; silicone resins with recurring organic groups such as polymethyl methacrylate; silica, such as colloidal silica, filled silicones; silicone resins derived from epoxy terminated silanes; and polyamide ester resins. These materials, among others, are harder and relatively more brittle than the underlying polycarbonate layer which they protect. It has been found, however, that in impact or shock-resistant laminates utilizing such a relatively hard and brittle coating along with the polycarbonate, the relatively brittle material cracks under impact causing damage to exposed objects behind the laminate. Such spalling occurs because of the so-called "notch sensitive" character of polycarbonates. Thus, if the overlying brittle layer is broken, the fracture lines propagate to the polycarbonate and act as "critical" notches causing the polycarbonates to fail in a brittle manner with little of the energy absorption typical of this normally impact-resistant material. The "notch sensitive" character of polycarbonates is greater, the lower the temperature.
One solution to this problem of notch sensitivity is described in British Pat. No. 1,504,198 which discloses that the notch sensitivity of the polycarbonate lamina coated with a relatively more brittle coating in impact shock resistant laminates is substantially reduced if the thickness of such a lamina is restricted to from 30 to less than 220 mils. The present invention provides another solution to the problem of notch sensitivity in impact and shock resistant laminates which utilize polycarbonate and an overlying relatively more brittle coating.